I believe in the Quake case they made all the walls transparent so you could see everyone coming. Which means they weren’t doing much fog of war at the time.
The engine has a flag that is mainly used to draw the player's weapon (which is rendered above the rest). Using this flag for players was the "engine compliant" way of making a wall hack.
Or in team fortress, replace the sniper's red aimpoint dot with a big model. They become easier to avoid.
Of course not great from large distance / players behind multiple walls but I think the server wouldn't send you the info of those guys anyway.